Gluing surfaces

[Dave Higham]

I wasn't sure where to put this, so if it needs moving, please feel free.

There has been quite a lot of discussion in the past, on various forums, on how to prepare surfaces for gluing. I think it was generally accepted that freshly planed or scraped surfaces are best. Is this still the case?

I ask because I just noticed this on the Stewmac site under 'Neck Construction Tips and Techniques'. It was on the same page as their email advert for bird's eye maple neck blanks.

https://www.stewmac.com/How-To/Online_R ... iques.html

"When gluing pieces of wood together, don't sand the two gluing surfaces too smooth. If the wood has come off of a jointer or planer or has been sanded finer than 180-grit, scuff-up the gluing surface with 80 or 120-grit sandpaper. This will improve adhesion and actually make the glue joint less visible. This is also useful when joining the two (or more) parts of a neck, an acoustic or solidbody's book-matched top or back, and any other glue joint".

Should someone have a word with them? The email was also advertising Al Carruth's Ultimate Scrapers. What does Al think?

[Chuck Tweedy]

hey Dave - long time no see - and I second your question. seems strange that Stew-Mac would be promoting that method.

[Mark Wybierala]

Three types of information, Fact, Myth, Lore. I believe that this idea is an example of incomplete lore that maybe could evolve into fact with some experimentation. Any advantage in joint strength would be dependent on the species and individual characteristics of the part. Close grained maple doesn't absorb liquid very deeply into the surface. I can imagine a planed surface with a less than optimal blade creating a polished surface and inhibiting glue absorption but it would be a rare occurrence. Certainly with non-porous materials like plastics, there is advantage to roughing up the surface. I don't see a problem with keeping this idea in your head when gluing joints but I don't think it's always applicable. Of equal value would be a practice of cleaning every joint surface with water to remove residual sawdust from pores.

[Barry Daniels]

I see one common situation where the advice is actually beneficial. If the planed surfaces were more than 24 hours old, then cleaning with sandpaper would expose fresh active surfaces that would accept glue more readily. How many people work fast enough that a freshly cut joint might not sit around for a couple of days, before they get around to glueing it up.

Keep in mind is that "freshly prepared joint" means different things to different people. I have read that in as little as 15 minutes the surface active sites can start to degrade. So I liken it to a fresh cut apple. They can sometimes turn brown in 15 minutes. I know this is an extreme analogy but it keeps me aware to not dawdle.

[Alan Carruth]

I was told that the 15 minute limit for a really 'fresh' surface comes from research done back in WW II by the Forest Products Lab. They also found around that same time that machine planed surfaces glued better than sanded ones. Aside from leaving dust behind, sanding chews up the surface and leaves loose fibers sticking up. Planing should leave a smooth surface. Scraping is a bit less good, as it tends to burnish down the surface fibers. Note that a dull machine planer/joiner can compress the wood and leave a surface that's less then optimal.

All of this backs up the notion that the main strength of a glue bond is chemical, not mechanical. The wood may not need to absorb anything, it just has to get wet, and 'toothing' either with a tooth plane or sandpaper probably does more harm than good.

[Mike Spector]

This is from the Technical Advisor at Franklin International:

Our work has shown that a smooth surface will always have higher strength than a rough surface. Two-hundred grit or higher sanding to get flat or tight-fitting joints works well.
Wood glues work by attaching to cellulose on the wood and the smoother (tighter) the joint, the less adhesive is needed to bond the surfaces. Less adhesive gives fewer areas of imperfections (bubbles, skips, dust and gaps) where stress can accumulate and cause glue line failure. Also, wood glues tend to be around 50% solids and therefore shrink when they dry. If the rough surface is too “gappy,” as the adhesive dries and shrinks, it will pull away from one surface or the other leaving gaps in the glue line, which again will concentrate force when the joint is stressed. This is why wood glues need to be clamped. Clamping keeps the surfaces in contact as the glue shrinks and dries.
A note of caution on smooth surfaces: Burnished areas may be smooth, but will not bond. Burnishing causes the cellulose to change chemical characteristics and thus not bond to the polyvinyl alcohol portion of the wood glue. This can be tested by putting a drop of water on the surface of the wood, if it doesn’t soak in, the surface is burnished or sealed and should be sanded until cleaned of the burnishing.
For hammer veneering, you can use either hot or liquid hide glue. I have read that roughing the surface of the substrate and veneer gives better strength. But our work shows that too much roughing of the surface can cause loose fibers and fiber tear which can weaken the bond. As above, the adhesive bonds to the cellulose so a roughed surface is not necessary, but as long as it doesn’t damage the wood surface to be bonded, it will produce good results on veneers.
Bob Behnke
Sr. Technical Specialist

[Brent Tobin]

The only adhesive that I know of that recommends scuffing is epoxy.
Some bad info on grain orientation as well.

Brent

[Alan Carruth]

Brent Tobin wrote:
"The only adhesive that I know of that recommends scuffing is epoxy."

Much of the information I got on surface energy came from an article in the Experimental Aircraft Association's 'Experimenter' magazine, which focuses on construction issues. It was from a detailed article on assembling fiberglass components, which make up a lot of the 'skin' in 'kit' airplanes these days (I'll note that some of these are very high performance machines). At any rate, these guys tend to be pretty fussy about construction details, as you can imagine: you don't want to worry about the glue lines when you're cruising along at altitude at 250 mph.

Many of these components come with 'peel plies': tapes that have been epoxied on along edges that are meant to be joined. These can be peeled away from the main assembly just before joining, to provide a fresh surface with high energy. The article does talk about sanding for the same purpose, but, iirc, recommends using fairly fine sandpaper to dull the surface, rather than 'toothing' with a coarser paper. Keep in mind here that they're using epoxy as a glue, which is essentially 100% solids, so that there's no worry about shrinkage as mentioned on the Franklin site. It's plausible that deep scratches could risk poor penetration by the adhesive, and coarse sanding tends to leave a lot of dust that can also get in the way.

[Dan Smith]

I work with veneer a lot.
My veneer vendor suggested lightly scuffing Maple prior to gluing.
Not sure why, but it may pertain only to veneer Maple.

[Alan Carruth]

Wood that has been sitting around for a while can have low surface energy. The spritz test will tell you. If the water tends to bead up, then some sort of surface dressing is in order. I like to scrape lightly, as it leaves a better gluing surface than sanding, but light sanding with not-too-coarse paper works too. You're not looking for big scratches from sanding, but rather a dull surface .

[Barry Daniels]

Some veneer is sliced off with a knife which burnishes the surface.